Na+-K+-Cl- co-transport in the intestine of a marine teleost.

Abstract

A key element in the absorption of salt and water by various epithelia is the presence, in the apical or brush border membrane, of a mechanism for directly coupling the inward movements of Na+ and Cl− (ref. 1). In this way the downhill gradient for Na+ from lumen to cell, which is maintained by the cell's Na+/K+ pump, can provide the energy for cellular accumulation and transepithelial transport of Cl−. A requirement for K+ at the serosal surface of the epithelium has long been accepted as essential to the operation of the Na+/K+ pump and therefore to salt absorption. Until very recently, however, the effect of luminal K+ on NaCl absorption has not been explored. We show here that luminal K+ stimulates NaCl absorption in the intestine of a marine teleost, the winter flounder, Pseudopleuronectes americanus. Furthermore, K+ uptake across the brush border membrane depends on the presence of both Na+ and Cl− in the bathing medium and is inhibited by furosemide, which also inhibits the coupled uptake of Na+ and Cl− (ref. 2). Thus, flounder intestine seems to have a Na+ −K+ −Cl− co-transport system similar to that demonstrated in erythrocytes3–5, cultured tumour cells6, cultured kidney cells7 and squid giant axon8. Electrophysiological studies of the thick ascending limb of Henle's loop in mammalian kidney9 and the diluting segment in amphibian kidney10 also indicate that luminal K+ is required for salt absorption. Co-transport of Na+, K+ and Cl− may therefore be characteristic of a variety of cells and tissues, including salt-absorbing epithelia.